1. Field of the Invention
The present invention relates to a mobile station which performs a frequency control for synchronizing the frequency of an internal clock signal in the mobile station to the frequency of a clock signal in a base station.
2. Description of the Related Art
For a mobile station to correctly demodulate a signal received from a base station, symbols in the received signal must appear at predetermined locations on a phase plane in accordance with a particular modulation scheme. To this end, the mobile station performs an automatic frequency control for synchronizing the frequency of an internal clock signal in the mobile station to the frequency of a clock signal in the base station.
A conventional automatic frequency control in a CDMA mobile station is performed only making use of an error in the frequency of an internal clock signal in the CDMA mobile station with respect to the frequency of a clock signal in a base station (cell/sector) to which the CDMA mobile station is currently in connection for standby, position registration, communication therewith, or the like. For this reason, in the event of hand-over for switching from a base station (cell/sector) in communication due to the CDMA mobile station being turned on or returning from an out-of-coverage area, fluctuations in reception power, and the like, the CDMA mobile station requires a long time to converge, to a certain range, a frequency error of the frequency of the internal clock signal in the CDMA mobile station with respect to the frequency of a clock signal in a base station (cell/sector), to which the CDMA mobile station is newly connected.
In addition, a conventional automatic frequency control for use in a CDMA mobile station employs a method of previously storing a TCXO (temperature-compensated crystal oscillator) control voltage for reducing a time required to converge a frequency error to a certain range when the CDMA mobile station is turned on or returns from an out-of-coverage area. This method stores in a memory a TCXO control voltage when a frequency error is below a certain threshold, i.e., when AFC (Automatic Frequency Control) is locked, such that the TCXO control voltage is read from the memory for use as an initial value for the TCXO control voltage when the CDMA mobile station is turned on or returns from an out-of-service range. However, this method can suffer from an instantaneous increase in the frequency error, on the contrary, when a plurality of base stations present errors in frequency among them.